The Conservative Party are claiming to be standing up for fair representation of these Parties, however it is clear that they are all to aware that they have strong opposition in the coming election.
Nothing a Party Leader says or does in the run up to a general election is unselfish. The only reason David Cameron could have for giving these smaller Parties a platform is to somehow keep him and his Party in power. How? By dividing those who don't want him there.
All of the parties are aware of the problems with First Past The Post voting, only to the main Parties they aren't problems - they're advantages.
Arguably one of the biggest factors in David Cameron becoming PM was Nick Clegg's stellar performance in the TV debates. In winning votes for himself, with notably left wing policies, Clegg took votes from people who would normally vote Labour, reducing the Labour vote enough to hand David Cameron a Win, if not a Majority.
David Cameron knows how he won the last election and is merely trying to recreate the same conditions this time round.
As for Karl McCartney? I think he's just avoiding having to answer questions about his vote against Equal Marriage.